Calendered elastomeric polymer-based compounds find application in many important industrial and commercial articles. Representative of such calendered elastomeric polymer compound articles, are roof sheeting, roof membranes, roof flashing, and, the like. The term compound may have several meanings. For purposes of this specification the term compound shall mean an elastomeric polymer based composition including other ingredients known to those of skill in the art, such as reinforcing fillers, plasticizers, oils, curatives, accelerators, processing aids, and the like.
In the manufacture of calendered articles, compounds of polymers and additives such as reinforcing fillers, extenders, waxes, accelerators, curatives, and the like, can, and often are, premixed in either a continuous mixer or an enclosed mixer such as a Banbury.RTM. mixer, and then the compound is placed on a calender for shaping into a useful article. The process of calendering is one which involves relatively little shear. If calender rolls are turning at the same speed the shear will generally be minimal and it will only be the shear created as the compound is forced to move in the space between the calender rolls. If the calendering rolls move at different speeds, then the shear is increased somewhat. The other common method of forming useful articles from elastomeric polymer compounds is extrusion. Extrusion differs generally from calendering in two significant ways; first is that the shear created on the polymer in the article-forming process is substantially higher, often 1 to 2 orders of magnitude higher, than the shear created by calendering; and the second important difference is that the temperature of an extrusion operation is generally higher than the temperature at which a calendering operation is carried out.
Due to the low shear, relatively low temperatures, and very large differences in the forces impinging on the material parallel and perpendicular to the direction of calendering, articles fabricated from a calendering process often can have large directional orientation. Ideally the directional orientation is largely isotropic. These small differences in orientation lead to relatively uniform physical properties measured in both the machine direction (generally the direction in which the elastomeric polymer and the calendering rolls are moving) and the transverse direction (the direction perpendicular to the machine direction). The relative uniformity of orientation leads to a lack of divergence, of many physical properties in one direction compared to the other direction and this is of interest and of high value in applications where directionally divergent physical properties would be detrimental. These application areas include roof sheeting, roofing membranes, roof flashing and the like. An imbalance, or substantial imbalance of physical properties parallel to and perpendicular to the direction of milling would lead to poor failure properties, such as tear resistance. The relative uniformity of physical properties, especially the failure properties, of an elastomeric polymer compound sheet made by a calendering operation, should preferably be substantially similar in all directions, including machine direction and transverse direction, both before and after curing or vulcanization. Typically the failure properties may be measured by a tear test, where the sheet or membrane is ruptured in a specific direction. In an ideal sheet, with substantially isotropic properties, the force needed to tear or rupture the sheet should generally be an invariant function of the direction of operation of the calendering process.
Another property of calendered materials that is valued most in products and applications produced is smoothness of the sheet produced. Smoothness of the calendered sheet generally arises from the ability of the elastomeric polymer compound to flow uniformly under the influence of the shear field of the calendering mill. Thus both smoothness and isotropic properties of the calendered sheet arise from the ability of the polymer compound to flow under the relatively weak shear field of the calendering mills. This ability to flow under the weak shear field is generally most important transverse to the direction of calendering. A polymer compound which exhibits such an ability to flow under such a weak field is termed processable. Processable compounds differ in their ability to respond to shear. More processable polymer compounds generally show isotropic physical properties and smoothness when calendered for shorter times, at lower temperatures, and at lower shear than less processable polymer compounds. Processability may be approximated, for the purpose of certain embodiments of our invention, by the viscosity of the elastomeric polymer compound in the shear zone of the calender. The ideal calendering membrane should be processable before vulcanization in terms of the description above, such that the calendering operation can run at a faster speed.
In calendering operations, the size of a sheet will be dictated by the width of the calender rolls. So, if a width larger than the calender roll width is more convenient or more economical for an end-user, the fabricator is left with several alternatives including gluing or adhesively laminating the strips. However, in roofing sheets such gluing or lamination is generally less effective, less strong, and therefore less acceptable than a process where the sheets are spliced together in their green state. After such a splice the sheet or membrane will have a wider interval between sections which have to be adhesively laminated after vulcanization. Typically, the spliced end products are wide sheets of ethylene, .alpha.-olefin, non-conjugated bicyclic diene elastomeric polymer compounds. These end product sheets of the membranes will generally be wider than the width of the calendering mills. These wider sheets are made by adhering, in an overlapped splice, the uncured sheets of the elastomeric polymer compound. Generally for this overlap splice to be successful, the adhesion in the uncured or green state of two or more of the calendered sheets is important.
In commercial operations, when splicing is accomplished with green, calendered, elastomeric polymer compounds, the splice is most often made by bringing two calendered sheets of a smooth polymer compound together at a time, and pressing or forcing them together for a short time at ambient temperature. The pressure exerted on polymer sheets during operations of this type are typically less than about 25 pounds per square inch (PSI) (172 KPa) and the time the pressure is exerted is in the range of about 5 seconds. These times and pressures will describe typical manufacturing conditions where the manufacturer is interested in both speed and maintenance of quality.
Another important property for calendered sheets, especially of the uncured or green compound is its tensile strength. This property is of importance to the fabricator or compounder because in the handling of sheets in the manufacturing operation, it is economically and physically impractical to have the sheet supported at all times in all locations. That is, there are usually distances between conveying devices where the uncured sheet is unsupported, and forces such as its own weight can temporarily deform and/or stretch the sheet, the deformation will generally be greater the lower the compound's green strength. Additionally, as the uncured polymer sheet moves over, across, and around various points in a manufacturing operation, the polymer sheet is subjected to elongation or stretching. The ability of a compound to resist such orientation or stretching and maintain its dimensional stability, may improve the final product's general homogeneity of properties in directional physical property tests. Also, such elongation or stretching could alter the thickness of the sheet thereby widening manufacturing tolerances, a generally unacceptable outcome.
The properties of spliced adhesion and green strength or tensile strength in the uncured state are additional constraints in the formation of a smooth sheet with isotropic (physical) properties.
It can be seen that the manufacturer of such a sheet could have competing needs from the green or uncured compound during the calendering and final spliced sheet formation. The competing needs occur because the peel adhesion principally arises from the ability of the macro-molecules in the elastomeric polymer compounds to flow and establish adhesion between two sheets. Generally this requires the diffusion viscosity of the polymer compound to be so low that the flow can be maintained under very gentle shear conditions which may be applied for a short period of time. On the other hand, high values of green strength or tensile strength which are desirable can generally be seen to come from the ability of the polymer compound to resist shear forces without substantial deformation and this implies a rather high viscosity. This inherent need for two seemingly opposing properties of a polymer compound have led to a number of years of substantially unsuccessful attempts to develop ethylene, .alpha.-olefin, diene monomer elastomeric polymers which simultaneously have good peel adhesion and good green strength.
There is a commercial need therefore for an elastomeric polymer or an elastomeric polymer compound based on the elastomeric polymer or a combination of elastomeric polymers, that will show a combination of excellent green strength and excellent peel adhesion during a calendering operation.